Model 202 Ozone Monitor Manual Rev. J-4
15
the user (see Section 6). For example, it may be desirable to provide a positive offset
by a known amount (e.g., 10 ppbv) if the analog output is being used for external data
logging since the analog output does not go negative below zero ppbv. Because of
noise and/or an inherent offset, some measured values will be below zero at very low
ozone mixing ratios or while zeroing the instrument with an external scrubber. Also,
the instrument zero may drift by a few ppbv over time. For this reason, frequent
zeroing of the instrument using an external ozone scrubber to determine the offset is
recommended (see Section 7). Any change in the slope (gain) of the instrument is
likely due to a serious problem such as contamination, an air leak, obstruction of air
flow, or loss of catalytic activity by the internal ozone scrubber, but it also can be
adjusted. Once the zero of the instrument is corrected, the slope may be adjusted so
that the instrument readout agrees with a standard ozone source or with the readout
from another instrument whose calibration is considered to be accurate.
To change the calibration parameters, select
Cfg
from the
Main Menu
:
Cfg Menu
D/T Cal I/O Unt
Now use the rotary select switch to select and click on
Cal
. The following submenu
will appear:
Cal Menu
Fm O3
Selecting
Fm
will display the Flow meter calibration factor as in the following example:
Fm Cal Menu
Fm= 0.87
The internal flow meter is calibrated at the factory during the production of the ozone
monitor. This is a multiplicative factor that will increase the flow rate if you increase
the value. Adjust this value to correct the flow value when comparing it to a calibrated
volumetric flow meter connected to the inlet of the instrument.
By selecting
O3
, the following submenu with the values of the current calibration
parameters will then appear:
O3 Cal Menu
Z=-2 S=1.01
Here Z is the offset applied (in this case
–2 ppbv) and S is the slope applied (in this
case 1.01). The value of Z is added to the measured ozone value, and the value of S
is then multiplied by the measured ozone value. For example, if the instrument reads
an average of 3 ppbv with the external scrubber in place, the value of Z should be set
to
–3. If after correction for the zero, the instrument consistently reads 2% low, the
value of S should be set to 1.02.
When the
Cal Menu
first appears, the
Z
will be underlined with a cursor. You may
rotate the Select switch to choose the calibration parameter
S
or
Z
. A single click on
S
or
Z
will select that parameter for change and activate a blinking cursor. Once
S
or
Z
is selected, its value can be changed by rotating the Select switch to the left or right.
